1. Field of the Invention
This invention relates to a wrist mechanism for an industrial robot or the like, and more particularly to improvements in or relating to a wrist mechanism for an industrial robot, a manipulator or the like wherein three wrist joints are constructed to make rocking, rocking and revolving motions successively from the driving side.
2. Description of the Related Art
Wrist mechanisms for industrial robots or the like having three wrist joints which are popularly employed are roughly divided into two types including such a type as shown in FIG. 5(a) wherein the three wrist joints of the wrist mechanism 100a make revolving, rocking and revolving motions successively from the driving source 80 side, and such another type as shown in FIG. 5(b) wherein the three wrist joints of the wrist mechanism 100b make rocking, rocking and revolving motions successively from the driving source 80 side.
Such a wrist mechanism 100a of the revolving-rocking-revolving motion type has been widely used because the structure thereof is comparatively simple.
However, where an arm 5a having the driving source 80 built therein and a revolving joint 81a, a first wrist element 1a, a rocking joint 82a, a second wrist element 2a and another revolving joint 83a connected to the arm 5a successively toward the driven side as well as a third wrist element 3a which serves as a tool receiving shaft are positioned on a straight line as viewed on a plane as shown in FIG. 5(a), the first wrist element 1a connected to the driven side of the revolving joint 81a and the third wrist element 3a connected to the driven side of the revolving joint 83a act as shafts which can be driven in the same directions (in the directions indicated by arrow marks 84 and 85 or arrow marks 86 and 87) at the same time.
Therefore, it is difficult to calculate an aimed point or an interpolation point which is necessitated for driving control of the robot, and it is impossible to determine which one of the two wrist elements 1a and 3a should be driven. Accordingly, control of the entire wrist mechanism 100a is disabled.
In order to prevent such an unpredictable and uncontrollable condition from taking place in a range of operation of the wrist mechanism 100a, the wrist mechanism 100a is suitably applied to a robot which is used in such an environment that the robot and a work (object to be worked) remain in a relatively stationary condition, such as where a work remains in a stationary condition or where a work is being transported while the entire robot which is carried, for example, on a bogie is moved to follow the work to reproduce an operation taught to the robot.
To the contrary, in the case of the wrist mechanism 100b of the rocking-rocking-revolving motion type shown in FIG. 5(b), such an uncontrollable region is entered when the rocking joint 82b is rotated at an angle of .+-.90 degrees so that the arm 5b and the third wrist element 3b are put into parallel positions to the axial directions thereof as shown therein.
However, since the operation at such an angle as mentioned above occurs proximate a terminal end of the range of operation of the wrist mechanism 100b, this does not matter at all practically.
Accordingly, such a wrist mechanism 100b as described above is suitable for processing a work carried by a series of conveyors, for example, for a coating, welding or sealing operation.
Such a wrist mechanism 100b of the rocking-rocking-revolving motion type as described above is disclosed, for example, in Japanese Patent Laid-Open No. 61-25796.
Generally, a backlash which takes place at a mutually meshing location of rotation transmitting parts has a bad influence between a reduction gear and a wrist element producing a "play" which reduced accuracy of the wrist mechanism. Accordingly, a component or components are necessitated for removing a backlash at such a mutually meshing location as described above.
To the contrary, between a driving source and the reduction gear, such backlash is multiplied by a reduction ratio (1/100 to 1/50 times) comparing with that on the wrist element side. Accordingly, the backlash is very small, and the bad influence thereof on operation of the wrist mechanism is comparatively small. Accordingly, countermeasures for removing a backlash at such a mutually meshing location are seldom taken.
As shown in FIGS. 6 to 9, the wrist mechanism 100b shown in FIG. 5(b) includes motors 88, 89 and 90 built therein adjacent a root portion of the horizontal arm 5b for driving the first to third wrist elements 1b, 2b and 3b, respectively. Reduction gears 91, 92 and 93 are disposed adjacent the motors 88, 89 and 90 so as to transmit rotation of the motors 88, 89 and 90 to the wrist elements 1b, 2b and 3b, respectively, at reduced speeds of 1/100 to 1/50.
Here, rotation transmitting routes from the motors 88, 89 and 90 to the wrist elements 1b, 2b and 3b are described in outline. At first as for the first wrist element 1b, high speed rotation of the motor 88 on the horizontal arm side is transmitted to a drive shaft 94 after the speed thereof is reduced by the reduction gear 91, and is further transmitted to the wrist side by way of a bevel gear wheel pair 95, a transmitting shaft 96 and a gear wheel 97 and then by way of a chain 98. Then, the rotation is transmitted to a gear wheel 99 on the wrist side so that the first wrist element 1b on which the gear wheel 99 is securely mounted is rocked in a direction perpendicular to the plane of FIG. 6.
Similarly, as for the second wrist element 2b, rotation of the motor 89 is transmitted to a gear wheel 101 by way of the reduction gear 92 and so forth and then transmitted by way of a chain 102 as well as gear wheels 103, 104, 105 and 106 to a bevel gear wheel pair 107. The rotation is further transmitted to a gear wheel 109 by way of a gear wheel 108 so that the second wrist element 2b on which the gear wheel 109 is securely mounted is rocked in a direction perpendicular to the plane of FIG. 7.
Further, as for the third wrist element 3b, rotation of the motor 90 reduced in speed by the reduction gear 93 is transmitted successively to a gear wheel 110, a chain 111 and another gear wheel 112 and further transmitted to a bevel gear wheel pair 116 by way of gear wheels 113, 114 and 115. Then, the rotation is transmitted to a bevel gear wheel pair 119 by way of gear wheels 117 and 118 so that the third wrist element 3b on which one of the bevel gear wheels of the bevel gear wheel pair 119 is securely mounted is revolved.
Such a wrist mechanism 100b for an industrial robot or the like as described above is superior in that a high degree of positioning accuracy can be attained because the mechanisms for driving the wrist elements are all controlled to make circular motions.
However, in such a conventional wrist mechanism for an industrial robot or the like as described above, reduction gears are disposed proximate motors adjacent a root portion of a horizontal arm and remote from the wrist mechanism. Further, power transmitting parts between the reduction gears and wrist elements are required to have a high strength so that they may withstand high torque, and besides a large number of parts are required because a component or components for removing a backlash are required. Accordingly, the conventional wrist mechanism is disadvantageous in that the wrist mechanism and the horizontal arm are increased in size and weight and that the mechanical rigidity is low.